The present invention relates to a nitrogen generation method and apparatus in which air is separated in a distillation column into nitrogen-rich vapor and oxygen-rich liquid fractions. More particularly, the present invention relates to such a method and apparatus in which oxygen-rich liquid, vaporized within a head condenser, is recompressed and reintroduced into the column and also, is in part, expanded with the performance of work which is in turn applied to the recompression. Still, even more particularly, the present invention relates to such a method and apparatus in which an auxiliary refrigerant stream is utilized to increase the amount of the work of expansion that can be applied to the recompression of the vaporized oxygen-rich liquid.
There are numerous prior art processes and apparatus in which air is distilled in a distillation column to produce a nitrogen-rich vapor which is taken as a product. In one type of air separation process and apparatus employing a single distillation column, air, after having been filtered, compressed and purified, is cooled in a main heat exchanger to a temperature suitable for its rectification. Thereafter, the air is introduced into the single column and separated into nitrogen-rich vapor and oxygen-rich liquid fractions. In order to reflux the column, a head condenser is employed in which oxygen-rich liquid is used to condense nitrogen-rich vapor. The vaporized oxygen-rich liquid is then recompressed and re-introduced into the column in order to increase nitrogen production. This compression can take place at a temperature of either the warm or cold ends of the main heat exchanger. Part of the vaporized rich liquid can be partially heated and then expanded with a performance of work. It would seem inviting to apply all this work of expansion to recompression of the vaporized rich liquid. However, for the case where compression occurs at the temperature of the cold end of the main heat exchanger, a heat of compression is produced which would have to be dissipated within the main heat exchanger. The end result would be that no net refrigeration would be made. Thus, a great proportion of the work of expansion must be rejected from the plant by way of an energy dissipative brake.
Typically, such plants as have been described above, make their entire product as a gas. In order to convert the product into a liquid, the product gas must be liquified in a separate liquefier. Such liquefaction is not accomplished without increased energy costs. At the same time, if high purity nitrogen is desired, the equipment involved in the liquefaction can act to contaminate the high purity nitrogen produced by the nitrogen generator. Thus, provision must be made for downstream cleaning of the liquid nitrogen if such liquid nitrogen is to be utilized in a high purity application.
As will be discussed, the present invention provides a nitrogen generation method and apparatus in which more of the work of expansion can be applied to the compression to enhance liquid nitrogen production in an energy efficient manner. Additionally, such liquid nitrogen production is accomplished without the use of a downstream liquefier.